In the 20th century, a vaccine for yellow fever was developed to protect people in regions where the disease still flourishes. Epidemics continued, however, during every decade of the century, and cases still surface to the present time.

The Panama Canal
Soon after the discovery that mosquitoes transmit yellow fever, Major William Crawford Gorgas put the information to work in Cuba and later in the Panama Canal Zone. A previous French attempt to build the canal ended in disaster when disease, particularly yellow fever and malaria, incapacitated thousands of project workers. Gorgas, the sanitation chief in Cuba during the U.S. occupation, fumigated against mosquitoes and rid the area of standing water, leading to a healthy workforce and successful project. But while the opening of the Panama Canal in 1914 eased travel throughout the world, it also increased the possibility of spreading disease.

Path to a Vaccine
After he became surgeon general of the Army in 1914, Gorgas met with Dr. Wickliffe Rose of the Rockefeller Foundation, which subsequently focused on eradicating yellow fever from the world. Rockefeller researchers traveled to yellow fever hotspots such as South America and Africa. Several of them died from the disease. One researcher, Dr. Alexander Mahaffy, discovered in 1927 that yellow fever could be transmitted and studied in animals. A Harvard scientist, Max Thieler, showed in 1930 that mice were susceptible to the yellow fever virus. Using mouse and chick embryos, Thieler later developed the most effective vaccine against yellow fever, called 17D, which immunized U.S. soldiers during World War II and became the world standard. The achievement won him the Nobel Prize in Medicine in 1951.

Outbreaks Continue
For the last 50 years, yellow fever has centered around South America and Africa, the only two places where it occurs today. Jungle yellow fever struck Central America in the 1950s and 1960s, and South America has experienced scattered outbreaks since then. Africa has seen the worst outbreaks, including epidemics in Central and East Africa between 1958 and 1966 that killed thousands of people. Hundreds died in a Ghanaian epidemic in the 1970s while Nigeria suffered thousands of fatalities from the disease in the 1980s. Other cases occurred in the 1990s, and in 2003, the World Health Organization cited new cases in West Africa and southern Sudan which caused at least 60 deaths. Immunization efforts in some French-speaking African countries between 1933 and 1961 resulted in the gradual disappearance of the disease there.

Current Status
Between 1996 and 2002, five people from the United States and Europe who traveled to South America or Africa contracted yellow fever and died. They were all unvaccinated. The Center for Disease Control and Prevention estimates that several hundred cases of yellow fever still occur each year, though because of underreporting the number could be much higher. Large populations remain unvaccinated and some health officials fear more instances of the disease. Vaccination, which is generally safe, and avoiding mosquito bites are the best preventive measures against yellow fever.